Environmental interests and an increasing demand for biofuels encourage fuel producers to employ more intensively renewable sources available for replacing petroleum-based feeds. In the manufacture of diesel fuels, the main interest has concentrated on vegetable oils and animal fats comprising triglycerides of fatty acids. Long, straight and mostly saturated hydrocarbon chains of fatty acids correspond chemically to the hydrocarbons present in diesel fuels. However, neat vegetable oils display inferior properties, particularly high viscosity and poor stability, and therefore their use in transportation fuels is limited.
Conventional approaches for converting vegetable oils or other natural materials containing fatty acids and fatty acid derivatives into liquid fuels comprise processes such as transesterification, catalytic hydrotreatment, hydrocracking, catalytic cracking without hydrogen and thermal cracking. Typically triglycerides, forming the main component in vegetable oils, are converted into the corresponding esters by the transesterification reaction with an alcohol in the presence of catalysts. The obtained product is fatty acid alkyl ester, most commonly fatty acid methyl ester (FAME). However, poor low-temperature properties of FAME, resulting from the straight chain nature of the molecule, limit its wider use in regions with colder climatic conditions. Thus double bonds are needed in order to create even bearable cold flow properties. Carbon-carbon double bonds and ester groups decrease the stability of fatty acid esters, which is a major disadvantage of transesterification technology. Further, it is also generally known that the presence of oxygen in esters results in undesired and higher emissions of NOx when compared to conventional diesel fuels.
Many biomass derived organic compounds could be suitable as fuels or their components, provided that their oxygen content is reduced. This is especially true with fatty acids derived both from animal and plant originating triglycerides. Oxygen can be removed by hydrodeoxygenation reactions, but the need of hydrogen is often excessive. The hydrodeoxygenation of oils and fats derived from biological material to hydrocarbons suitable as diesel fuel components is typically carried out in the presence of hydrogen and a catalyst under controlled hydroprocessing conditions.
During hydrodeoxygenation oxogroups are reacted with hydrogen and removed through formation of water. The hydrodeoxygenation reaction requires relatively high amounts of hydrogen. Due to the highly exothermic reactions the control of reaction heat is extremely important. Unnecessarily high reaction temperature, insufficient control of reaction temperature and unnecessarily low hydrogen availability in the feed stream cause formation of unwanted side reaction products and coking of catalyst. Side reactions, such as cracking, polymerisation, ketonisation, cyclisation and aromatisation decrease the yield and have negative impact on the properties of the product, such as the diesel fraction. Unsaturated feeds and free fatty acids in triglyceridic oils and fats derived from biological materials may also promote the formation of heavy molecular weight compounds. U.S. Pat. No. 5,705,722 describes a process for the production of diesel fuel additives by conversion of oils and fats derived from biological material into saturated hydrocarbons under hydroprocessing conditions. The process operates at high temperatures and produces n-paraffins and other hydrocarbons. The product has high cetane number but poor cold flow properties, which limit the amount of product that can be blended in conventional diesel fuel in the summer time and prevent its use during the winter time.
A two-step process is disclosed in patent FI 100248 for producing middle distillates from vegetable oils by hydrodeoxygenating fatty acids or triglycerides of vegetable oil origin using commercial sulphur removal catalysts to give n-paraffins, followed by isomerisation of said n-paraffins using metal containing molecule sieves or zeolites to obtain branched-chain paraffins. The hydrotreating is carried out at rather high reaction temperatures of 330-450° C. Hydrodeoxygenating fatty acids at those temperatures leads to shortened catalyst life resulting from coking and formation of side products.
EP 1 396 531 describes an alternative process containing at least two steps, the first one being a hydrodeoxygenation step and the second one being a hydroisomerisation step utilizing counter-current flow principle. Biological raw material containing fatty acids and/or fatty acid esters serves as the feedstock. The process may also comprise prehydrogenation and optional stripping steps.
Decarboxylation of carboxylic acids to hydrocarbons by contacting carboxylic acids with heterogeneous catalysts was suggested by Maier, W. F. et al.: Chemische Berichte (1982), 115(2), 808-12. Maier et al. tested Ni/Al2O3 and Pd/SiO2 catalysts for decarboxylation of several carboxylic acids. During the reaction the vapors of the reactant were passed through a catalytic bed together with hydrogen. Hexane represented the main product of the decarboxylation of the tested compound heptanoic acid.
Biological raw materials often contain several impurities, such as metal compounds, organic nitrogen, sulphur or phosphorus containing compounds, said compounds being known catalyst inhibitors and catalyst poisons inevitably reducing the service life of catalysts and necessitating more frequent catalyst regeneration or replacing. Metals in oils and fats derived from biological material tend to build up on catalyst surfaces and they change the activity of the catalyst. Blocking of active sites of catalysts by metals typically decreases the activity of catalysts. Metals may promote some side reactions too.
Hydrolysis of triglycerides produces also diglycerides and monoglycerides, which are partially hydrolyzed products. Diglycerides and monoglycerides are surface-active compounds, which can form emulsions and make liquid/liquid separations of water and oil more difficult. Oils and fats derived from biological material may also contain other glyceride-like surface-active impurities like phospholipids containing phosphorus in their structures, such as lecithin. Phospholipids are gum like materials, which can be harmful to catalysts. Natural oils and fats also contain non-glyceride components. These are among others waxes, sterols, tocopherols and carotenoids, some metals and organic sulphur compounds as well as organic nitrogen compounds. These compounds can be harmful to catalysts or pose other problems in processing.
Oils and fats derived from biological material may contain free fatty acids, which are formed during processing of oils and fats through hydrolysis of triglycerides. Free fatty acids are a class of problematic components in bio oils and fats, their typical content being between 0 and 30% by weight. Free fatty acids are corrosive in their nature, they can attack the materials of process units and catalysts and, in the presence of metal impurities they can promote side reactions like formation of metal carboxylates. Due to the free fatty acids contained in oils and fats derived from biological material, the formation of heavy molecular weight compounds during processing is significantly increased when compared to triglyceridic feedstock having only low amounts of free fatty acids, typically below 1% by weight.
Fatty acid composition and the size and saturation degree of fatty acids may vary considerably in feeds of different origin. Melting point of oils and fats derived from biological material is mainly a consequence of saturation degree. Fats are more saturated than liquid oils and in this respect they need less hydrogen for the hydrogenation of double bonds. Double bonds in fatty acid chains contribute also to different kinds of side reactions, such as oligomerisation, polymerization, cyclisation, aromatisation and cracking reactions, which deactivate the catalyst, increase hydrogen consumption and reduce diesel yield.
Deoxygenation of plant oils and fats and animal oils and fats with hydrogen requires rather much hydrogen and at the same time releases significant amounts of heat. Heat is produced from deoxygenation reactions and from double bond hydrogenation. Different feedstocks produce significantly different amounts of reaction heat. The variation in reaction heat produced is mainly dependent of double bond hydrogenation. The average amount of double bonds in a triglyceride molecule can vary from about 1.5 to over 5 depending on the source of oil or fat.
Predominantly paraffinic or olefinic Fischer-Tropsch products obtained from synthesis gas derived from biomass contain also variable amounts of oxygenates, such as alcohols, ethers, carboxylic acids and esters of carboxylic acids. The amount and nature of said oxygenates depend on the carbon number range of the selected Fischer-Tropsch fraction and the Fischer-Tropsch process used in the processing. Particularly for fuel applications it is desirable to reduce the amounts of oxygenates.
Carbon monoxide has been used for the reduction of iron and other ores for a long time, and also in the field of organic chemistry reduction reactions with carbon monoxide are known.
In the reference: Thomson, W. J. and Laine, R. M., Homogeneous catalytic reduction of benzaldehyde with carbon monoxide and water. Applications of the water gas shift reaction. ACS Symposium Series (1981) 152 (Catal. Act. Carbon Monoxide), 133-45, the use of Rh6(CO)16, Fe3(CO)12 and Ru3(CO)12 as catalysts for studying the kinetics of benzaldehyde reductions with CO—H2O was disclosed.
According to literature, carbon monoxide reacts with alcohols, ethers and esters to give carboxylic acids. Suitable catalysts are rhodium and cobalt catalysts in the presence of iodine. This reaction is for example the basis for the commercial production of acetic acid, as presented in the following formula:CH3OH+CO→CH3COOH
Based on the above it can be seen that there exits an evident need for an alternative method for the deoxygenation of materials of biological origin and also for a method for decreasing the consumption of hydrogen when converting biomass derived feedstock to hydrocarbons, suitable as bio fuel.